Delayed Second Cell Cleavage in Human Early Embryo Is Associated With Adverse Developmental Competence and Pregnancy...

To investigate the impact of the second division timing on pregnancy outcome after in vitro fertilization.

In this unpaired retrospective study, 16,696 embryos from 6571 patients were analyzed. Patients were divided into four groups according to the number of blastomeres observed on day 2 (44 ± 1 h) and day 3 (68 ± 1 h) of development. Blastocyst formation rates and pregnancy outcomes were compared among the groups. Propensity Score Matching was applied to eliminate discrepancies among the baseline data of the different groups.

The blastocyst formation rates in Group A (delayed second cleavage) and Group B (delayed second cleavage and early compaction) were significantly lower than that of Group C (normal cleavage) (P < 0.001). For both day 3 single embryo transfer and frozen single blastocyst transfer, the clinical pregnancy and live birth rates in Group A were comparable to those of Group C. For day 3 double embryo transfer, the biochemical pregnancy, clinical pregnancy, and live birth rates following the combined transfer of embryos from Groups A and C were significantly lower than those obtained when two embryos from Group C were transferred.

Embryos with delayed second cell cleavage exhibited reduced blastocyst formation rates and resulted in poorer pregnancy outcomes even when the third cleavage was completed on time. These findings indicate that the kinetics of embryo development are closely associated with developmental competence and pregnancy outcomes.